Aryl cyanate and/or diepoxide and hydroxymethylated phenolic or hydroxystyrene resin

ABSTRACT

An adhesive for bonding a die of an integrated circuit chip comprises an aryl cyanate (ester) resin alone and/or a diepoxide resin admixed with a hydroxymethylated phenolic resin or hydroxymethylated poly(hydroxystyrene) which can further contain an electrically or thermally conductive filler.

The application is a continuation of application Ser. No. 08/585,343,filed Jan. 11, 1996, abandoned.

TECHNICAL FIELD

This invention generally relates to adhesives for bonding a die to asurface. More particularly, the invention relates to conductiveadhesives that reduce or eliminate resin bleed and provide enhanced dieto substrate bond integrity when exposed to elevated temperature andhumidity.

BACKGROUND OF THE INVENTION

Integrated circuit chips, which are also referred to as dies, are oftenassembled in electronic packages that include the die and a substrate.The substrate has a gold plated or unmetallized ceramic surface to whichthe die is mechanically attached using an organic polymer-basedadhesive. The surface is the floor of a cavity which is defined by oneor more tiers that surround the surface. Alternatively, the surface isan entire side of the substrate, i.e., there is no cavity. Surroundingthe surface, either on the tier(s) or co-planar with the surface, arewire bond pads to which the die is electrically connected to thesubstrate by wires that run from the die to the wire bond pads. Inhybrid applications, wire bonding results in interconnections that arechip-to-chip, chip-to-passive component or the like. Representativedevices that use the electronic packages are computers, automobiles,calculators and other consumer and industrial products.

High performance reliability of the mechanical bonding of the die to thesubstrate requires that adhesive: has good adhesion to the die and thesurface; shows no resin-bleed; has good high temperature stability; andmaintains adhesive bond integrity during temperature and humidity stressexposure.

The adhesive is often a silver filled epoxy or cyanate ester. A commonproblem with conventional silver filled epoxies is that the epoxy resinseparates, or "bleeds", out of the adhesive during die attachment and/orcuring of the adhesive. Resin bleed out is related to surface porosity,surface wettability and adhesive type, Ireland, Int. J. HybridMicroelectronics, Vol 5, No 1, pp 1-4 (February 1982). Bleed out can bereduced by implementing a bake of the metallized substrate at elevatedtemperatures and reduced pressure prior to bonding to change the surfaceproperties The die must be attached within a few hours which is notalways possible. The bake process does not work satisfactorily onnon-metallized surfaces. Epoxy based adhesives are unable to withstandhigh temperatures, e.g., greater than 200-220° C., and therefore areunsuitable for high temperature applications.

The cyanate ester adhesives also have the problem of resin bleed out onmetallized and unmetallized ceramic surfaces. Silver filled cyanateester-based adhesives have a problem of adhesion degradation duringtemperature and humidity stress exposure at 85°/85% humidity whichlimits their use to packages that are hermetically sealed to keep outmoisture.

An adhesive exhibiting reduced bleed out and which does not exhibit theabove shortcomings is highly desirable.

SUMMARY OF THE INVENTION

This invention provides conductive adhesives that are characterized bythe reduction or elimination of resin bleed out, good high temperaturestability and maintenance of die to substrate bond integrity undertemperature and humidity stressing at 85° C./85% humidity. The adhesivesinclude a resin that is an aryl dicyanate monomer based cyanate esterresin or an epoxy resin and an additive that is a functionalizedoligomeric/polymeric phenolic resin having the formula: ##STR1## whereinX is H, CH₃ or C₂ H₅ and n is 3 or 4. Other additives in this categoryinclude tetrahydropyranyl protected bisphenol A and the like. Theadhesive can also include a conductive material, e.g., silver flakes,and a catalyst for curing the cyanate ester to a thermoset polymermatrix. Alternative adhesives include blends of the cyanate ester resinand the epoxy resin and the functionalized oligomeric/polymeric phenolicadditive of formula I.

Numerous other advantages and features of the present invention willbecome readily apparent from the following detailed description of thepreferred embodiments and the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The adhesive of the present invention includes a resin that is a cyanateester resin or epoxy resin and an additive which is a functionalizedoligomeric/polymeric phenolic resin having the following formula:##STR2## wherein X is H, CH₃ or C₂ H₅ and n is 3 or 4. A conductivematerial in admixture with the resin and additive makes the adhesiveconductive. The conductive material is dispersed in the resin or resinand additive mixture to produce a dispersion or paste.

The resin is preferably an aryl dicyanate monomer based cyanate esterhaving 5-20% of the monomeric functionality converted to triazine ringsor a prepolymer, an epoxy or admixture thereof. The cyanate ester resinis primarily made up of the monomer with only a minor amount of thetrimerized polymer being present. Preferred resins are liquid at aboutroom temperature, i.e., about 20° to about 35° C.; albeit solid resinscan be used if a suitable solvent is used to dissolve the solid.

Representative aryl dicyanate-based prepolymers include the prepolymercommercially available from Ciba-Geigy Corporation, Hawthorne, N.Y.,under the designation Arocy L10, the liquid dicyanate monomer ethylidenebis 4,1-phenylene dicyanate (described in U.S. Pat. No. 4,839,442 toCraig) and in a paper by Shimp and Craig, presented at the 34thInternational Symposium in Reno, May 8-11, 1989, the dicyanate withcycloaliphatic bridging group commercially available from Dow ChemicalCo. under the designation XU-71787, dicyanates of the type NCO--(AR)--R₁--(AR)--OCN where R₁ is selected from the group of O, SO₂ and C(CH₃)₂and AR represents an aryl group (which is commonly referred to as ArocyB10), liquid dicyanate ester monomer derived from 4,4'- 1,3 phenylenebis (1-methylethyl-idene bisphenylcyanate)!; 4,4-dicyanato-2,2-diphenylperfluoropropane (Arocy F10); bis(4-cyanatophenyl)ether; tetra-o-methylbisphenol F dicyanate; like cyanate ester resins based on bisphenol Awhich are commercially available from Ciba-Geigy and Rhone Poulenc, Inc,Louisville, Ky. and mixtures thereof.

The epoxy resin is an aromatic cycloaliphatic, aliphatic or bisphenol Abased diepoxide.

Representative epoxy resins include: 3,4-epoxy cyclohexylmethyl-3,4-epoxy cyclohexane carboxylate, commercially available fromUnion Carbide under the designation ERL 4221; bis-3,4-epoxy cyclohexyladipate; bis-epoxide derived from 4-vinyl-1-cyclohexene commerciallyavailable from Union Carbide under the designation ERL 4206;bis-(3,4-epoxycyclohexyl) adipate commercially available from UnionCarbide under the designation ERL 4289; bis-glycidyl ethers such as2,2-bis (4-glycidoxyphenyl)propane; 1,4-butane diol diglycidyl ether;diglcidyl ether of bisphenol F; the like and mixtures thereof.

These epoxides are used in conjunction with conventional anhydridehardeners such as hexahydrophthalic anhydride, methyl hexahydrophthalicanhydride, maleic anhydride and the like.

The reactive additive is selected to react with the cyanate resin or theepoxy resin and also undergo a self-condensation reaction during curing.The additive is a prepolymer or polymer and preferably is misciblewithout organic solvent with the liquid dicyanate monomer/prepolymer andthe epoxy resin. The number average molecular weight of the additive ispreferably in the range of about 1,000 to about 8,000 daltons.

Representative additives include hydroxymethyl novolak resin (i.e.,hydroxymethyl-cresol formaldehyde resin); hydroxymethylpoly-(hydroxystyrene), hydroxymethyl α-methyl-poly-(hydroxy styrene);acetal protected phenolic resins such as polyhydroxystyrene and novolakresins partially protected as tetrahydropyranyl or tetrahydrofuranylether groups; low molecular weight (about 2,000 to 4,000 daltons)poly(hydroxystyrene); hydroxypolyimides; siloxane polyimides;polyimides; the like and mixtures thereof. Common elements of theseadditives are that they are soluble in amounts in the range of about5-15 wt % with Arocy L10 based on the total weight of the additive andthe Arocy L10 at room temperature, have reactive groups such that duringcuring the additives interact with the cyanate group and also canundergo self-condensation to form higher molecular weightinterpenetrating networks. Low molecular weight, i.e., a molecularweight of about 2,000 to about 4,000 daltons was defined as 2,000-4,000!poly(p-hydroxy styrene) can be used without requiring the use of anorganic solvent. A higher percentage of hydroxymethylation of thepoly(p-hydroxystyrene) or novolak produces a resin that is insoluble inthe cyanate resin.

Similarly, novolak resin with a molecular weight of about 1,000 to about2,000 daltons hydroxymethylated to introduce up to 15% --CH₂ OH whenused as an additive dissolves in Arocy L10 at a weight ratio of up to6:1, resin to hydroxymethylated novolak resin.

The hydroxymethyl cresol formaldehyde resin (i.e., the hydroxymethylnovolak) and hydroxymethyl poly-p-hydroxy styrene have a degree ofhydroxymethylation in the range of about 5 to about 20, preferably about5 to about 15, percent based on the molar equivalent of formaldehydeused for a molar equivalent of the monomer unit of novolak orpoly(hydroxy-styrene). If the degree of hydroxymethylation is too highthe adhesive cannot be dissolved in Arocy L10 without an organicsolvent. If it is too low, the additive is less effective ineliminating/reducing resin bleed out and providing resistance to thedegradative effects caused by high temperature and humidity.

Acetal group protected phenolic resins, specificallypoly(p-hydroxystyrene) and novolak, having 10 to about 30 percent of thehydroxyl groups converted to tetrahydropyranyl ether ortetrahydrofuraryl ether, dissolves in cyanate esters in an amount up toabout 20 wt % based on the combined weight of the resin and cyanateester without using an organic solvent. Acetal protected phenolic resinsare described in Hayashi et al, ACS Polymeric Materials Science andTechnology Vol 61, p447 (1989) and Hesp et al, Journal of AppliedPolymer Science Vol 42 pp877-883 (1991), although Hesp describe theiruse in the unrelated photoresist art.

The resin and additive are admixed together in a resin:additive weightratio in the range of about 95:5 to about 80:20.

The adhesive preferably also includes an electrically or thermallyconductive filler. The electrically conductive filler is preferably ahighly conductive metal, such as silver, gold, copper and nickel, andpreferably is in the form of flakes. The thermally conductive fillerscan be AlN, SiO₂, SiC, BN, the like and mixtures thereof. The weightratio of resin and adhesive admixture to filler is preferably in therange of about 15:85 to about 50:50.

The adhesive can also include conventional hardeners and catalysts suchas organometallic catalysts, e.g., stannous octoate and dibutyl-tindilaurate, metal naphthenates, the like and mixtures thereof, asdescribed in U.S. Pat. No. 4,604,452 to Shimp andacetylacetonate/alkylphenol curing catalysts as described in U.S. Pat.No. 4,785,075 to Shimp, and optionally a conventional surfactant.

The adhesive is prepared according to standard techniques for preparingpolymer and filler dispersions which involves blending the filler in theresin or the resin and additive using a blender or mixer and mixing toproduce a substantially homogeneous dispersion which is then degassedand stored at a temperature of -20° C. For bonding the die to thesubstrate in a manual operation, the adhesive is dispensed as a bead orlines on the surface and the die is placed thereon with gentle pressure.Alternatively, the die can be bonded using a conventional tool. Thesubstrate and die assembly is subsequently heated to a temperature inthe range of about 150 to about 200° C. for a time period in the rangeof about 10 to about 45 minutes to cure the adhesive.

The following examples are given by way of illustration and notlimitation.

EXAMPLE 1 Conductive Adhesive with Arocy L10 Resin and HydroxymethylPoly(P-Hydroxystryrene) Additive

In a suitable vessel, 0.35 grams (gms) of hydroxymethylpoly(p-hydroxystyrene) having 10% hydroxymethylation was dissolved in4.2 gms Arocy L10 at a temperature of 55° C. with stirring to obtain aresin and additive mixture that includes 8 wt % additive. The resultingmixture was cooled to room temperature and 25 milligrams (mg) of acatalyst solution containing 6 wt % copper acetyl acetonate in 94 wt %nonylphenol was introduced into the vessel and mixed. Then, 13.5 gms ofsilver flakes having an average size of less than 10 microns wasintroduced with mixing. Mixing continued for 30 minutes with an inertgas blanket to produce the adhesive which was a homogeneous dispersion.The adhesive is degassed prior to being stored in an air tight containerat a temperature less than -20° C. for an extended time period. Prior touse, the adhesive is thawed to room temperature before opening thecontainer to prevent absorption of the moisture by the adhesive.

EXAMPLE 2 Attachment of Dies to Substrates

The adhesive of EXAMPLE 1 was used to attach dies to ceramic substrates.One of the substrate surfaces to which dies were attached was goldplated and another was unmetallized ceramic. A conventional bead of theadhesive was dispensed on the surface. A 12×7 millimeter die was placedon the adhesive and slight pressure applied until a fillet was visible.The die, substrate and adhesive were placed in an oven having a nitrogengas purge at a temperature of 170° C. for a time period of about 15-20minutes to cure the adhesive, form a strong bond between the die and thesurface and produce a die and substrate assembly.

There was no evidence of resin bleed out after the adhesive wasdispensed or cured. The absence of bleed out was confirmed by surfaceanalysis using x-ray photoelectron spectroscopy of the surface. Aconventional die shear test showed that the shear strength was greaterthan 10 kilograms (kg) for the 12×7 mm die for both the gold plated andceramic surfaces. To determine the effect of thermal shock, the assemblywas subjected to 1,000 thermal cycles of -65° to +150° C. followed bythe shear test. The test results for the shear test after the thermalstress showed no reduction in shear strength. To determine the effect oftemperature and humidity, the assembly was exposed to conditions of 85°C. and 85 % humidity for two weeks followed by the shear test. The testresults showed no discernable difference in shear strength after theexposure to elevated temperature and humidity.

EXAMPLE 3 Comparative Conductive Adhesive with Arocy L10 having 3.5 Wt %Hydroxymethyl-Poly(P-Hydroxystyrene) Additive

In a suitable vessel, 0.36 gms of hydroxymethyl-poly(p-hydroxystyrenewith 8% hydroxymethylation was dissolved in 10.5 gms of Arocy L10 withmixing to produce a resin and additive mixture having about 3.5 wt %additive. Then, 50 mg of a catalyst solution including 6 wt % copperacetyl acetonate in 94 wt % nonylphenyl was introduced into the vesseland mixed. Then, 33 gms of silver flakes having an average size of lessthat 10 microns were introduced with mixing. Mixing continued for 30minutes with an inert gas blanket to produce a comparative adhesive thatwas a homogeneous dispersion.

The comparative adhesive was subjected to the tests described in EXAMPLE2. The tests showed that the comparative example had resin bleed out onboth the gold plated and ceramic surface. The shear strength was greaterthan 10 kg.

EXAMPLE 4 Conductive Adhesive with Cyanate Ester Resin and Epoxy ResinBlend Containing Hydroxymethyl Poly(P-Hydroxystyrene) Additive

In a suitable vessel, 12.1 gm of a solution (prepared by dissolving 4.6gms hexahydrophthalic anhydride in 7.5 gm of the cycloaliphaticdiepoxide ERL 4221 at a temperature of 60° to 70° C. followed by coolingto room temperature) was mixed with 2.5 gm hydroxymethylpoly(p-hydroxystyrene) (10% hydroxymethylation) and 4.2 gm of thecyanate ester resin Arocy L10 with an inert gas blanket. Then, 50 mg ofbenzyl dimethyl amine, and 50 mg of a catalyst solution containing 6 wt% copper acetyl acetonate in 94 wt % nonylphenyl were introduced intothe vessel and mixed. Then, 50 gms of silver flakes having an averagesize of less that 10 microns were introduced with mixing. Mixingcontinued for 30 minutes with an inert gas blanket to produce theadhesive that was a homogeneous dispersion.

The adhesive was tested as described in EXAMPLE 2. The test resultsshowed no significant bleed out on the gold plated or ceramic surfaces.The shear strength was greater than 10 kg. After each of the thermalshock test and the temperature and humidity test, the shear strength wasnot degraded.

EXAMPLE 5 Conductive Adhesive with Arocy L10 and Tetrahydrofuranyl EtherProtected Poly(P-Hydroxystyrene) Additive

The partially protected poly(p-hydroxystyrene) was prepared by thereaction of poly(p-hydroxystyrene) having a molecular weight of about6,000 daltons with 2,3-dihydrofuran according to known procedures toobtain the additive having about 20% of the hydroxyl groups converted tothe tetrahydrofuranyl derivative. A conductive adhesive using the aboveadditive in place of the additive of EXAMPLE 1 was prepared using theprocedure, dure, components and amounts described in EXAMPLE 1. Theadhesive was tested according to the tests described in EXAMPLE 2. Diesheer strength, thermal shock, and temperature and humidity exposuretests showed satisfactory results.

EXAMPLE 6 Hydroxymethylation of Poly(P-Hydroxy Styrene)

In a suitable vessel, 25 gm of poly(p-hydroxystyrene) having a molecularweight of 6,000 daltons was dissolved in 230 ml in 1N NaOH solution. Tothis solution was added 1.5 ml formalin solution (40% aqueous solutionof formaldehyde) drop wise with stirring to produce a mixture. Themixture was kept at 40° C. with stirring for 3 days after which it wastreated with 1 equivalent of oxalic acid and precipitated product wasisolated, washed repeatedly with H₂ O and dried under vacuum to obtainabout 23 gms of off-white solid. The pH is preferably neutral to preventpremature cross-linking.

EXAMPLE 7 Conductive Adhesive with Poly(P-Hydroxystrene)

Poly(p-hydroxystyrene) of mw 2000 could also be dissolved in Arocy L10in an amount up to 20%. The conductive adhesive therefrom showed reducedbleed than a comparative adhesive made with cyanate ester alone withoutthe additive.

EXAMPLE 8 Hydroxymethylated Novolak and Conductive Adhesive with This asan Additive with Arocy L10

A 10% hydroxymethylated novolak was prepared by following the proceduredescribed for poly(p-hydroxystyrene) described in EXAMPLE 6 andsubstituting an equal amount of novolak for the poly(p-hydroxystyrene).A conductive adhesive was prepared using this additive and the cyanateester resin Arocy L10 and the procedure described in EXAMPLE 1. Theresults of tests described in EXAMPLE 2 showed no evidence of resinbleed and thermal shock and temperature and humidity stress exposurecaused no change in shear strength.

The adhesive does not exhibit bleed out so there is no contamination ofwire bond pads by the adhesive, results in a bond that exhibits goodresistance to temperature and humidity which permits the adhesive to beused in packages that are not hermetically sealed and is stable at hightemperatures.

It presently is theorized that the above-described advantages areachieved because of the combination of the resin and the additiveresults in them being in a more stable dispersion with the conductivematerial; and that the additive reacts with any low molecular weightfractions of the resin that may be present and also self condensesduring high temperature cure to form high molecular weight polymers sothat the resin does not separate.

This invention has been described in terms of specific embodiments setforth in detail. It should be understood, however, that theseembodiments are presented by way of illustration only, and that theinvention is not necessarily limited thereto. Modifications andvariations within the spirit and scope of the claims that follow will bereadily apparent from this disclosure, as those skilled in the art willappreciate.

We claim:
 1. An adhesive prepared from a resin composition, the resincomposition consisting of:a resin consisting of an aryl cyanate esterresin or an aryl cyanate resin, and a member selected from the groupconsisting of cycloaliphatic and aliphatic diepoxide resins andbisphenol A diepoxide resins; and an additive a consisting essentiallyof a hydroxymethyl functionalized oligomeric/polymeric phenolic resinhaving the formula: ##STR3## wherein X is H, CH₃ or C₂ H₅ , and whereinthe additive has a degree of hydroxymethylation in the range of about 5to about 15 percent based on the molar equivalent of the monomer unit ofnovolak or poly(hydroxy-styrene), the resin and the additive being anadmixture.
 2. An adhesive prepared from a resin composition, the resincomposition consisting of:a resin consisting of an aryl cyanate esterresin or an aryl cyanate resin, and a member selected from the groupconsisting of cycloaliphatic and aliphatic diepoxide resins andbisphenol A diepoxide resins; and an additive consisting of ahydroxymethyl functionalized oligomeric/polymeric phenolic resin havingthe formula: ##STR4## wherein X is H, CH₃ or C₂ H₅, and wherein theadditive has a degree of hydroxymethylation in the range of about 5 toabout 15 percent based on the molar equivalent of the monomer unit ofnovolak or poly(hydroxy-styrene), the resin and the additive being anadmixture.
 3. An adhesive prepared from a resin composition, the resincomposition consisting essentially of:a resin consisting of an arylcyanate ester resin or an aryl cyanate resin, and a member selected fromthe group consisting of cycloaliphatic and aliphatic diepoxide resinsand bisphenol A diepoxide resins; and an additive consisting essentiallyof a hydroxymethyl functionalized oligomeric/polymeric phenolic resinhaving the formula: ##STR5## wherein X is H, CH₃ or C₂ H₅, and whereinthe additive has a degree of hydroxymethylation in the range of about 5to about 15 percent based on the molar equivalent of the monomer unit ofnovolak or poly(hvdroxy-styrene), the resin and the additive being anadmixture.
 4. The adhesive according to claims 1, 2 or 3 wherein theresin is a liquid resin at room temperature.
 5. The adhesive accordingto claims 1, 2 or 3 wherein the resin is the cyanate ester resin.
 6. Theadhesive according to claims 1, 2 or 3 wherein the resin is a diepoxyresin selected from the group consisting of:3,4-epoxy cyclohexylmethyl-3,4-epoxy cyclohexane carboxylate; bis-3,4-epoxy cyclohexyladipate; bis-epoxide derived from 4-vinyl-1-cyclohexene;bis-(3,4-epoxycyclohexyl) adipate; 2,2-bis (4-glycidoxyphenyl)propane;1,4-butane diol diglycidyl ether; diglycidyl ether of bisphenol F; andmixtures thereof.
 7. The additive according to claims 1, 2 or 3 whereinthe additive has a molecular weight in the range of about 1,000 to about8,000 daltons.
 8. The adhesive according to claims 1, 2 or 3 wherein theadditive is a hydroxymethylated novolak or poly(p-hydroxystyrene) havinga degree of hydroxymethylation in the range of about 8 to about 15percent based on the molar equivalent of formaldehyde used for a molarequivalent of the monomer unit of novolak or poly(hydroxy-styrene). 9.The adhesive according to claims 1, 2 or 3 wherein the weight ratio ofresin to additive is in the range of about 95:5 to about 80:20.
 10. Theadhesive according to claims 1, 2 or 3 further comprising, in admixture,a filler.
 11. The adhesive of claim 10 wherein the filler is present ina weight ratio of resin or resin and additive to filler in the range ofabout 15:85 to about 50:50.
 12. The adhesive of claim 10 wherein thefiller is a conductive filler.
 13. The adhesive of claim 10 wherein thefiller is metal flakes.
 14. The adhesive according to claims 1, 2 or 3wherein the cyanate ester resin is selected from the group consisting ofthe prepolymer from the liquid dicyanate monomer ethylidene bis(4,1-phenylene dicyanate); dicyanates of the type NCO--(AR)--R₁--(AR)--OCN where R₁ is selected from the group of O, SO₂ and C(CH₃)₂,and Ar represents an aryl group liquid dicyanate ester monomer derivedfrom 4,4'- 1,3 phenylene bi(1-methylethylidene bis-phenylcyanate)!;4,4-dicyanato-2,2-diphenyl perfluoropropane; bis(4-cyanatophenyl)ether;tetra-o-methyl bisphenol F dicyanate; cyanate ester resins based onbisphenol A and mixtures thereof.
 15. The adhesive according to claims1, 2 or 3 wherein the additive formula is ##STR6##
 16. The adhesiveaccording to claims 1, 2 or 3 wherein the additive formula is
 17. Theadhesive according to claims 1, 2 or 3 wherein the first resin isfurther characterized by the absence of a solvent.
 18. The adhesive ofaccording to claims 1, 2 or 3 wherein the additive is selected from thegroup consisting of, hydroxymethyl-cresol formaldehyde resin,hydroxymethyl novolak resin other than the hydroxymethyl-cresolformaldehyde resin, hydroxymethyl poly-(hydroxystyrene) andhydroxymethyl alpha-methyl-poly(hydroxystyrene).